Many medical patients suffering from any one of a variety of lung ailments are often prescribed supplemental oxygen therapy so that the patient could breath oxygen-enriched air throughout the day and sometimes throughout the night. Earlier supplemental oxygen therapy employed a nasal cannula system operably connected between a tank of compressed oxygen and the patient's nose. Oxygen was continuously delivered to the patient throughout the patient's entire breathing cycle. This method of continuously delivering oxygen to the patient throughout the patient's breathing cycle was considered wasteful because much of the oxygen dissipated into the ambient air environment. Better methods of delivering oxygen to the patient were later developed which included improved equipment that would only deliver oxygen to the patient during the inhalation phase of the patient's breathing cycle. Usually, this improved equipment employed a demand valve which opened to deliver supplemental oxygen to the patient only when the patient inhaled. Numerous types of demand valves are well known in the prior art.
One such demand valve is described in U.S. Pat. No. 5,360,000 to Carter. This demand valve is compact, simplified and totally pneumatic. The demand valve which is coupled between a source of pressurized gas such as oxygen and the patient includes a valve body having a gas flow passageway and pneumatically-coupled sensing and slave diaphragms. The slave diaphragm is interposed in the gas flow passageway and prevents gas from flowing during the exhalation phase of the patient's respiratory cycle. During inhalation, which is sensed by a sensing diaphragm, the slave diaphragm moves to open the gas flow passageway, thus permitting flow of gas to the patient. Although effective in delivering gas to a patient upon demand, this demand valve has an inherent problem. When the patient inhales to cause delivery of oxygen to patient, oxygen is also vented into the ambient air environment for as long as the slave diaphragm remains opened. This leads to wastage of oxygen which is the very problem that demand valves were designed to prevent.
Furthermore, this demand valve has an inherent deficiency of delivering gas to the patient in a continuous flow stream upon and during the inhalation phase. Unfortunately, the air remaining in the patient's respiratory passageway i.e. the nasal cavity and the throat, is first taken into the lungs upon inhalation. The oxygen-enriched air then follows the remaining air and only approximately one-half of the oxygen-enriched air ever reaches the lungs. The remaining one-half of the oxygen-enriched air remains in the patient's respiratory passageway during the waning moments of inhalation and is the first to be exhaled therefrom during exhalation. It would be beneficial to the patient if this air remaining in the patient's respiratory passageway after exhalation could be purged or otherwise enriched with oxygen before it is inhaled. Such an approach is utilized in U.S. Pat. No. 4,686,974 to Sato et al.
There is a need in the industry to provide a pneumatically-operated gas demand apparatus which can control delivery of oxygen to the recipient/patient as the recipient inhales and exhales while minimizing wastage of oxygen. It would be advantageous of this pneumatically-operated gas demand apparatus can deliver a high-flow pulse of oxygen to the recipient/patient upon commencement of the inhalation phase of the patient's breathing cycle. Such a high-flow pulse of oxygen delivered upon commencement of the inhalation phase would enrich the air remaining in the patient's respiratory passageway upon inhalation and, simultaneously therewith, purge some of this air therefrom before being inhaled. It would also be advantageous if this pneumatically-operated gas demand apparatus can deliver a continuous flow of oxygen immediately after delivery of the pulse of high-flow oxygen and throughout the remaining portion of inhalation.
U.S. Pat. No. 5,666,945 to Davenport, the disclosure of which is incorporated herein by reference, describes a pneumatically-operated gas demand apparatus which overcomes many of the deficiencies of prior devices. The Davenport apparatus includes cooperating supply and sensing valves in interruptible fluid communication between a recipient (or patient) and at least a first source of pressurized gas. The supply valve includes a supply valve housing with a first diaphragm member disposed therein. Similarly, the sensing valve includes a sensing valve housing and a second diaphragm member disposed therein. The Davenport apparatus is constructed such that, when the recipient inhales, the second diaphragm member assumes a flow-causing position and the first diaphragm member assumes a flow-supplying position whereby pressurized respiratory gas is delivered to the recipient. When the recipient exhales, the second diaphragm member assumes a flow-stopping position and the first diaphragm member assumes a flow-blocking position, thereby preventing delivery of the respiratory gas to the recipient.
The pneumatically-operated gas demand apparatus of Davenport also includes a bolus chamber structure, a supply orifice element and a pilot orifice element. The bolus chamber defining a bolus chamber therein is disposed between and in fluid communication with a regulator mechanism and a supply chamber region of the supply valve. The supply orifice element having a supply orifice formed therethrough is disposed between the regulator mechanism and the bolus chamber structure. The pilot orifice element having a pilot orifice extending therethrough is disposed between a source of pressurized respiratory gas and the supply valve.
The bolus chamber functions as a repository or accumulator for a volume of pressurized respiratory gas which is discharged during inhalation and recharged during exhalation by the recipient. The bolus chamber enables the apparatus to deliver a high-flow pulse of oxygen to the recipient upon commencement of the inhalation phase of the recipient's breathing cycle. The high-flow oxygen pulse advantageously enriches the air remaining in the recipient's airway upon inhalation and, simultaneously therewith, purges some of the air from the recipient's respiratory passageway. The Davenport device also delivers a continuous flow of oxygen immediately after delivery of the pulse of high-flow oxygen and for the remaining portion of inhalation whereby the recipient receives oxygen enriched respiratory gas throughout inspiration.
The intermittent gas delivery device of Davenport may also be used with a nebulizer. Pursuant to this modality, the high-flow pulse of oxygen delivered from the bolus chamber generates a fine mist of medicament-containing aerosol within the nebulizer which is inhaled by the recipient. The mist may thereafter be followed with a flow of pressurized respiratory gas for the remainder of inhalation.
In the Davenport device, the pulse volume is proportional to the supplied flow rate. Such a system works well in circumstances where the ratio of highest to lowest recipient demand flow rate is on the order of about three to about four to one. However, many patients have rather expansive demand flow ranges. Under these circumstances, the required flow ratio (which is an empirical ratio of the maximum to minimum demand flow rates) may become quite large. The flow requirements for some recipients, for example, may range from as low as about 0.5 l pm (liters per minute) for sedentary persons to as high as about 6 l pm for persons under physical stress. This maximum to minimum flow ratio would in turn require a pressure range of up to about 12:1 or more. Such a broad band of demand flow requirements impacts design and operation of the Davenport system in several significant ways.
First, the regulator mechanism must employ a high pressure regulator to reduce the high pressure supply gas to a workable level and a low pressure regulator to service the broad range of flow and pressure delivered by the system. The high pressure regulator must be capable of supplying respiratory gas at the higher pressure regions, and the supply (or pilot) valve must be reinforced to accommodate this higher pressure. In so doing, the dynamic performance of the supply valve ("on" and "off" timing, leakage flow, and the like) may be detrimentally affected. Second, an extremely small pilot orifice must be provided between the source of pressurized respiratory gas and a control region of the supply valve to ensure proper operation of the sensing valve. Third, the operating range of the low pressure regulator must be quite broad which may result in errors in the low pressure regulator and its adjustment mechanism that may become substantial at lower operational settings.
An advantage exists, therefore, for a pneumatically-operated gas demand apparatus capable of providing a high-flow pulse of pressurized respiratory gas upon commencement of the inhalation phase of a recipient's respiratory cycle and which reliably operates in flow ranges from as low as about 0.5 l pm to as high as 6 l pm or more. The apparatus also preferably should be usable with a nebulizer to generate and deliver a medicament-containing aerosol to a recipient on demand.